Long – Term Rotational Stability of Some Isolated Radio Pulsars.
Comparison with earlier measurements has shown that long-term period fluctuations are probably dominated by recovery from (mostly unseen) glitches and timing noise. The relationship observed between changes in period & the period derivative (pulsar spin-down rate) is as expected since as the pulsar spin is being braked by any of the spin-down mechanisms, it should equally affect the period of spin.
However since this fluctuation/spindown is not detectable in some pulsars or is very small (especially millisecond pulsars), it makes pulsars suitable for high precision measurements over time span of months and years despite timing noise. The spindown of most pulsars is also dominated by a quasi periodic phenomena and this is probably telling us something about the interior of the neutron star.
Despite the fact that the most widely accepted model predicts a braking index of n = 3(i.e a constant value for braking index) it is obvious that most of the observed braking indices deviate appreciably from the expected value of 3, some having low negative values and some having high positive values, pointing to the fact that the existing pulsar spin-down models may have to be revisited.
The high correlation between the braking indices and some of the pulsar parameters like age and surface magnetic field as shown in Figures 3.8 (a) and (b) respectively is another example of the discrepancy between model prediction and observation since n is supposed to be constant independent of any variable as predicted by the pure magnetic dipole model.
It all began in 1967 when then-graduate student Joycelyn Bell saw it as a simple scratch on a piece of paper made by a chart recorder that was recording radio signals from the outer space. Fortunately, she realized that the scratch was clearly contrasting with known astronomical radio sources such as galaxies and again did not quite look like those produced by Earth – based radio interference.
This astonishing discovery was made using an instrument called the “Interplanetary Scintillation Array” built specially to detect short time scale intensity fluctuations and was sensitive to low flux density. The object which was emitting regular pulsations every 1.33 seconds was called LGM-1 (Little Green Man – 1) (Hewish et al., 1968), since it was envisioned that pulsations could come from an intelligent form of extraterrestrial life.
The mystery about the pulsating source took an interesting turn when Bell found three more sources with regular pulsations and periodicity of the order of one second. LGM-1 was then renamed CP 1919, after its right ascension (19h 19m), while the other three were CP 1133, CP 0834 and CP 0950 (Hewish et al., 1968). It was followed by a second paper reporting details about the three other pulsating radio sources and providing further timing results of CP 1919 (Pilkington et al. 1968).
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